1. Field of the Invention
The present invention relates to optical fiber amplifiers.
2. Description of the Prior Art
Optical fiber amplifiers are used at regular intervals in optical fiber transmission systems to compensate line losses. An optical fiber amplifier comprises a fiber doped with erbium, for example, and pumping means. The population inversion achieved by pumping amplifies the signal.
Various bands are used by optical fiber transmission systems, particularly in terrestrial transmission systems. The expression xe2x80x9cC-bandxe2x80x9d refers to the range of wavelengths from 1 529 nm to 1 565 nm and the expression xe2x80x9cL-bandxe2x80x9d refers to the range of wavelengths from 1 569 nm to 1 603 nm.
A problem associated with this type of optical fiber amplifier is that of noise. Power is lost because of amplified spontaneous emission (ASE). This noise propagates in the fiber in both directions. Amplified spontaneous emission noise occurs in particular at wavelengths at which the emission effective cross section is maximum (for example in the 1.5 xcexcm window in the case of an erbium-doped fiber). Also, the higher the population inversion, the higher the noise level. This is the case in particular at the end of the doped fiber adjacent the pumping means, where the pumping ratio is highest.
J. Massicott et al., xe2x80x9cLow noise operation of Er3+ doped silica fibre amplifier around 1.6 xcexcmxe2x80x9d, Electron. Lett., 1992, 20, p. 1924-1925 proposes the use of an erbium-doped fiber amplifier for signals in a band from 1 570 nm to 1 605 nm. The article explains that a population inversion of the order of 60% is suitable for using erbium-doped fiber as an amplifier in the band centered on 1 550 nm. On the other hand, a population inversion of around 35% enables use of the fiber as an amplifier in the band from 1 570 nm to 1 605 nm. The article proposes the use of two pumps at two different wavelengths to obtain the population inversion. The 1.48 xcexcm main pump has a power rating of 82 mW and is used to obtain a high population inversion. The 1.555 xcexcm auxiliary pump has a power rating of 1 mW and is used to saturate the amplifier and to reduce the population inversion to 35%.
Y. Sun et al., xe2x80x9c80 nm ultrawideband erbium-doped silica fibre amplifierxe2x80x9d, Electron. Lett., 1997, 23, p. 1965-1967 proposes an erbium-doped fiber amplifier which amplifies signals in the C-band and in the L-band. The amplifier has a first section which is common to both bands and a second section in which the signals in the two bands are amplified separately in two separate branches. Wideband Bragg gratings and circulators integrated into the fiber and reflecting the C-band are respectively used to multiplex and demultiplex the two bands before and after the second section. The use of two amplification stages in the amplification branch for the L-band is proposed; the first stage uses a 980 nm pump with a power rating of 20 dBm and the second stage uses a 980 nm pump with a power rating of 25 dBm and a saturation signal at 1 592 nm, with a saturation signal input power of the order of xe2x88x924 dBm.
J. Massicott et al., xe2x80x9cHigh gain, broadband 1.6 xcexcm Er3+ doped silica fibre amplifierxe2x80x9d, Electron. Lett., 1990, 20, p. 1645-1646 proposes amplifying signals at around 1 600 nm in a doped fiber pumped at 1 555 nm. The fiber proposed is a SiO2xe2x80x94Al2O3xe2x80x94GeO2 core fiber with a core diameter of 5.5 xcexcm and a xcex94n of 0.015. A fiber of this kind has a cut-off wavelength of the order of 1 200 nm.
The problem of optimizing the gain arises in prior art L-band amplifiers. In particular, using 1 480 nm or 980 nm pumps causes a high population inversion in the doped fiber in the vicinity of the pump injection coupler; there is then a high gain in this part; the amplified spontaneous emission noise is therefore also high. This causes energy losses. The length of the doped fiber can be varied to reduce the average population inversion for use in the L-band but cannot improve the efficiency of the amplifier.
The use of a 1 550 nm pump in the configuration proposed in the 1990 article by J. Massicott et al. also gives rise to the problem of managing noise and confining the signal and the pump within the core of the fiber.
The invention proposes a solution to the above new problems. It proposes an L-band amplifier which limits the effect of amplified spontaneous emission noise. It also proposes a transmission system using that amplifier.
To be more precise, the invention proposes an L-band optical amplifier including a doped optical fiber which has a cut-off wavelength not less than 1 535 nm and means for coupling a pump into the fiber. In one embodiment the pump supplies light in the C-band at a wavelength greater than the cut-off wavelength of the fiber.
The cut-off wavelength of the fiber is advantageously greater than 1 540 nm and the pump can provide at least 10% of the pumping energy and preferably at least 40% of the pumping energy.
In another embodiment the pump has a power rating of at least 10 mW.
The pump can be a co-directional pump or a contra-directional pump.
In another embodiment the amplifier includes means for coupling into the fiber a second pump supplying light at a wavelength less than the wavelengths of the C-band. For example, the second pump can have a wavelength of 980 nm or 1 480 nm.
The invention also proposes an optical fiber transmission system including an amplifier of the above kind and a source of L-band signals.
The invention also proposes an optical amplification method for an L-band signal, including injecting the L-band signal into a doped optical fiber having a cut-off wavelength not less than 1 535 nm and pumping the signal.
The signal is preferably pumped with light in the C-band at a wavelength greater than the cut-off wavelength of the fiber.
The cut-off wavelength of the fiber is advantageously greater than 1 540 nm and the light in the C-band can supply at least 10% of the pumping energy and preferably at least 40% of the pumping energy.
The pumping can be co-directional pumping or contra-directional pumping.
Another embodiment of the invention proposes pumping with light at a wavelength less than the wavelengths of the C-band, for example at a wavelength of 980 nm or 1 480 nm.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention, which description is given by way of example and with reference to the accompanying diagrammatic drawing.